The power amplifier used for radio communication is required to have high power efficiency. In general, however, linearity and power efficiency of the power amplifier have mutually contradictory characteristics. To achieve a balance between the linearity and the power efficiency, various distortion compensation methods have been proposed.
A predistorter has been known as one of the above-described distortion compensation methods. The predistorter is a method for obtaining a desired signal without distortion in an output of the power amplifier by previously adding a characteristic that is reverse to a distortion characteristic of an amplifier with respect to an input signal of the power amplifier. Furthermore, various methods have been proposed as a predistorter that compensates nonlinear distortion (amplitude distortion and phase distortion) of the power amplifier and hysteresis that is called a memory effect (see, for example, Japanese Laid-Open Patent Publication No. Hei 09-69733 and Japanese Laid-Open Patent Publication No. 2000-78037).
FIG. 1 illustrates a configuration diagram of an example of a conventional predistorter. In FIG. 1, transmission data I and Q are supplied to a terminal 1. The transmission data I and Q are supplied to a multiplier 2, an address generating unit 3, and a subtractor 4.
Based on an increase in the amplitude and the phase of the transmission data I and Q, the address generating unit 3 generates addresses (a one-dimensional address and a two-dimensional address) of a distortion compensation coefficient generating unit 5. A distortion compensation coefficient is read out from the distortion compensation coefficient generating unit 5 by these addresses and is then supplied to the multiplier 2.
The multiplier 2 performs complex multiplication of the transmission data I and Q by the distortion compensation coefficient. The output of the multiplier 2 is converted into an analog signal by a DA converter 6 and is then supplied to a quadrature modulator 7 to be quadrature-modulated. The radio frequency quadrature-modulated signal that is output from the quadrature modulator 7 is power-amplified by a power amplifier 8 and is then output from a terminal 10 through a directional coupler 9.
Some of the radio frequency quadrature-modulated signals are taken out from the directional coupler 9 and are then frequency-converted into intermediate frequency (IF) signals by a frequency converter 12. The IF signal is converted into a digital signal by an AD converter 13 and is then supplied to a quadrature demodulator 14. The quadrature demodulator 14 performs quadrature demodulation of the digital IF signal. The demodulated data I and Q are supplied to the subtractor 4 and a coefficient updating unit 15.
The subtractor 4 calculates a difference (error) between the transmission data I and Q from the terminal 1 and the demodulated data I and Q and then supplies the difference to the coefficient updating unit 15. There is a delay unit (not illustrated) between the terminal 1 and the subtractor 4. The delay unit performs time adjustment between the transmission data I and Q and the demodulated data I and Q.
Based on the distortion compensation coefficient read out from the distortion compensation coefficient generating unit 5, the difference, and the demodulated data I and Q, the coefficient updating unit 15 calculates an updated distortion compensation coefficient. Next, a new distortion compensation coefficient, obtained by adding the updated distortion compensation coefficient and the distortion compensation coefficient read out from the distortion compensation coefficient generating unit 5, is supplied to the distortion compensation coefficient generating unit 5. Accordingly, the distortion compensation coefficient of the distortion compensation coefficient generating unit 5 is updated.
FIG. 2 and FIG. 3 illustrate configuration diagrams of examples of the address generating unit 3 in a conventional predistorter. In FIG. 2, a power calculating unit 3a calculates a power value based on the transmission data I and Q. A subtractor 3c obtains a differential power between a power value output from the powΔer calculating unit 3a and a power value obtained by delaying this power value by a one clock delay unit 3b. The power value is output as a one-dimensional address, and the differential power is output as a two-dimensional address.
In FIG. 3, the power calculating unit 3a calculates a power value based on the transmission data I and Q. A differentiation/integration circuit unit 3d calculates a differentiation value or an integration value of the power value output from the power calculating unit 3a. The power value is output as a one-dimensional address, and the differentiation value or the integration value is output as a two-dimensional address.
FIG. 4 illustrates a configuration diagram of another example of the conventional predistorter. In FIG. 4, the transmission data I and Q are supplied to a terminal 21. The transmission data I and Q are supplied to a multiplier 22, a delay unit 23, a subtractor 24, and a series method distortion compensation unit 25. The delay unit 23 delays the transmission data I and Q by a specified time to supply the transmission data I and Q as delay data to the series method distortion compensation unit 25.
The series method distortion compensation unit 25 generates a distortion compensation coefficient by using a Volterra series or the like based on the transmission data I and Q and the delay data, and then supplies the distortion compensation coefficient to the multiplier 22.
The multiplier 22 performs the complex multiplication of the transmission data I and Q by the distortion compensation coefficient. The output of the multiplier 22 is converted into an analog signal by a DA converter 26 and is then supplied to a quadrature modulator 27 to be quadrature-modulated. The radio frequency quadrature-modulated signal that is output from the quadrature modulator 27 is power-amplified by the power amplifier 28 and is then output from a terminal 30 through a directional coupler 29.
Some of the radio frequency quadrature-modulated signals are taken out from the directional coupler 29 and are then frequency-converted into IF signals by a frequency convertor 32. The IF signal is converted into a digital signal by an AD converter 33 and is then supplied to a quadrature demodulator 34. The quadrature demodulator 34 performs the quadrature demodulation of the digital IF signal. The demodulated data I and Q are supplied to the subtractor 24 and a coefficient updating unit 35.
The subtractor 24 calculates the difference between the transmission data I and Q and the demodulated data I and Q from the terminal 21 and then supplies the difference to the coefficient updating unit 15. There is a delay unit (not illustrated) between the terminal 21 and the subtractor 24. The delay unit performs the time adjustment between the transmission data I and Q and the demodulated data I and Q.
Based on the distortion compensation coefficient generated by the series method distortion compensation unit 25, the difference, and the demodulated data I and Q, the coefficient updating unit 35 calculates an updated distortion compensation coefficient. Next, a new distortion compensation coefficient, obtained by adding the updated distortion compensation coefficient, the distortion compensation coefficient generated by the series method distortion compensation unit 25, and the distortion compensation coefficient output from the series method distortion compensation unit 25, is supplied to the series method distortion compensation unit 25. Accordingly, the distortion compensation coefficient of the series method distortion compensation unit 25 is updated.
Hysteresis of the power amplifier is a phenomenon in which a present signal is influenced by a past signal. The degree of the influence of the hysteresis may vary. As for the conventional predistorter, a delay amount in case of generating a delay signal depending on the degree of the influence of the hysteresis is fixed, and there was no idea that the delay amount is changed. Because of this, there has been a problem that the hysteresis of the power amplifier is not compensated sufficiently.